1. Field of the Invention
The present invention relates to a photoelectric encoder for use in precise measurements and method of manufacturing a scale as an element of the photoelectric encoder.
2. Description of the Related Art
A photo electric encoder (hereinafter, it may also be simply referred to as the “encoder”) has been employed in the art for precise measurements of linear displacements and angular displacements. The encoder may be mounted on a coordinate measuring machine and an image-measuring instrument. A brief description is given to a measurement by the photo electric encoder as follows.
A light source and a photoreceiver are moved relative to a scale. Light from the light source is applied to an optical grating on the scale alternatively through a plurality of index gratings having different spatial phases. A plurality of (for example, four) resultant sinusoidal optical signals with different phases are received at a plurality of photodiodes (photoreceiver) corresponding to respective phases and photoelectrically converted to generate electric signals, which are employed to measure displacements such as a linear displacement.
Typically, encoders are classified into (a) a transmissive type that employs light applied to the optical grating to transmit through the scale in measurements; and (b) a reflective type that employs light applied to the optical grating and reflected from the scale in measurements. Chromium has been well used as a material for the reflective optical grating.
The reflective optical grating has a structure provided with projections and depressions regularly in an optical reflective layer such as a chromium layer. Particularly, in the case of a diffraction grating scale, unevenness in processed depths caused on forming projections and depressions in the optical reflective layer enlarges variations in in-plane distribution and repeatability. The variation in in-plane distribution is a associated with differences in processed depth and arrayed pitch depending on locations in one optical grating. The variation in repeatability is associated with differences in processed depth and grating pitch among a plurality of optical gratings. As the optical signal to be received at the photoreceiver to generated from the optical grating on the scale, the above variations prevent improvements in the measurement accuracy.
Methods for forming uniform processed depth in the optical grating include, for example, the following two. One is a technology that utilizes a difference in etching rate between a silicon substrate and a silicon oxide layer. With an etching stopper of the silicon substrate, the silicon oxide layer formed on the silicon substrate is selectively etched to form projections and depressions therein, which serve as a diffraction grating (optical grating) (for example, see JP-A 7-113905, FIG. 1 and Paragraph [0043]).
Another is a technology that provides a triple-layered structure including upper chromium, silicon oxide and lower chromium layers. With a mask of the upper chromium layer and an etching stopper of the lower chromium layer, the silicon oxide layer is selectively etched to form projections and depressions therein, which serve as a phase grating (optical grating) (for example, see JP-A 8-286020, FIGS. 1–3 and Paragraphs [0010]–[0013]).